Construction machines known in the related art include the one disclosed in, for instance, Japanese Patent No. 2633095, which has a traveling hydraulic motor driven with the discharged oil from a variable displacement hydraulic pump which is driven by an engine, and is capable of controlling an engine rotation speed in response to an operation of a travel pedal as well as allowing the vehicle to travel by controlling the quantity of the pressure oil supplied to the hydraulic motor in response to the operation of the travel pedal.
In the construction machine disclosed in the above-mentioned publication, the input torque is controlled on the basis of the following speed sensing control. That is, a target torque to prevent an engine stall is calculated based on a deviation between an actual engine rotation speed detected by a rotation speed sensor and a target rotation speed corresponding to a position of a governor lever of the engine, and then a target pump displacement angle is calculated based on the target torque so as to control the pump displacement angle. When calculating the target torque, the operation is executed only to increase the input torque but not to decrease the input torque. In this manner, the displacement angle of the hydraulic pump is maintained greater than or equal to a predetermined value, and as a result, smooth acceleration can be secured.
In recent years, engines designed to handle exhaust gas have been used in order to suppress the generation of black smoke. In the engine designed to handle exhaust gas, the full load performance curve is set such that the engine output torque in the low speed range is smaller than a value for conventional engines. More particularly, the maximum output torque of the engine is shifted to the high speed range, and the torque is increased gradually from the low speed range to the middle speed range whereas the torque is increased sharply from the middle speed range to the high speed range. As a result, the fuel consumption in the low speed range can be reduced so as to suppress the generation of black smoke.
The following problems would occur if the input torque was controlled as described in the above-mentioned publication with such an engine that copes with exhaust gas. That is, since the input torque is not controlled to decrease in the construction machine according to the above-mentioned publication, the input torque may exceed the engine output torque when the vehicle is starting traveling or climbing uphill and the travel load becomes greater, which may cause an engine stall to occur.